2 Reasons I can think of for him to not be Nostr only.

1. Nostr's user base is incredibly small, and you would be preaching to the choir. The people that understand this are already here. The message is already understood. The larger networks of the current popular platforms give a massively larger uninformed audience and reach, compared to Nostr and it's tiny niche.

2. He is making money from this. Zaps are great, but once again, the minuscule number of users isn't going to pay the bills.

Sometimes aware pragmatism makes more sense than blind idealism.